The function of an image signal detection device, i.e., an input head of an image reproducing system, is very important since image signal detection has a great influence on image processing and on the quality of the output image. A conventional scanner, as shown in FIG. 1, exposes light on an original picture drum A with a beam producer for obtaining a reflected beam 2 or with a beam producer for obtaining a transmitted beam 2'. An enlarged image of the original picture is obtained by picking up the reflected or transmitted beam via a pickup lens 3, and detecting the enlarged image through a slit 4 with a detection device 5 such as a photo-multiplier or photo-diodes. Rotation of the original picture 1 fixed on the drum means A below the pickup lens 3 establishes scanning along a main scanning line and this routine is carried out for subsequent main scanning lines. Between successive main scanning lines for a sub-scanning length defined as the distance perpendicular to the main scanning direction; the sub-scanning length is established in accordance with the magnification rate of the pickup lens 3 and gap width of the slit 4. This sub-scanning length should be controllable since the screen ruling number may be changed or the magnification in sub-scanning direction between the input side (fixed) and the output side (variable) may be changed. To put this method into practice, a conventional scanner employs multiple slits which have several properly selected gap widths or a slit having a gap width which can be mechanically gradually or step-wise varied.
However, the former method requires multiple slits which have several gap widths, while the latter method requires a complicated mechanical system.